A NAND-type flash memory is a semiconductor memory that needs erase processing before performing writing. In writing/erasing of data in the NAND-type flash memory, electrons are injected/removed in/from a floating gate by applying a high voltage between a substrate and a control gate. It is known that when the writing/erasing is performed many times, a gate oxide film around the floating gate is degraded, which adversely affects writing/erasing characteristics closely related to a life of the NAND-type flash memory. Therefore, it is desired to correctly recognize the life of the NAND-type flash memory.